1. Field of the Invention
The present invention relates to a distributed feedback (DFB) semiconductor laser which can change its polarization mode of output light according to its stimulated condition and can be used as a signal light source for optical transmission and the like, and to a driving method of the DFB semiconductor laser.
2. Related Background Art
In a conventional oscillation polarization mode selective DFB semiconductor laser as disclosed in Japanese Patent Application Laid-Open No. 7(1995)-162088, for example, the relation between the wavelength dependency of gains created in its active layer and Bragg wavelengths determined by the pitch and the like of its diffraction grating is controlled, and a multi-electrode structure is adopted for that purpose.
As an oscillation polarization mode selective dynamic single mode semiconductor laser, the following device has been developed and proposed. The oscillation polarization mode selective device has a structure that can be modulated by a digital signal which is produced by superposing a minute-amplitude digital signal on a bias injection current. The device is a DFB semiconductor laser in which a distributed reflector of a grating is introduced into a semiconductor laser resonator or cavity and wavelength selectivity of the grating is utilized. In the device, strain is introduced into an active layer of a quantum well structure, or its Bragg wavelength is located at a position shorter than a peak wavelength of a gain spectrum, so that gains for transverse electric (TE) mode and transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near an oscillation threshold. Further, a plurality of electrodes are arranged and currents are unevenly injected through those electrodes. An equivalent refractive index of the cavity is unevenly distributed by the uneven current injection, and oscillation occurs in one of the TE mode and the TM mode and at a wavelength which satisfies a phase matching condition and takes a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary a competitive relation of the phase condition between the TE mode and the TM mode, the oscillation polarization mode and wavelength of the device can be switched.
In that semiconductor device, an anti reflection coating is provided on one end facet to asymmetrically employ effects of the uneven current injection into its output-side portion and its modulation-electrode portion. Alternatively, lengths of the electrodes are made different from each other to introduce a structural asymmetry.
Further, Japanese Patent Application Laid-Open No. 2(1990)-117190 discloses a semiconductor laser apparatus which has two serially- or parallel-arranged semiconductor devices that primarily generate or amplify light waves in a predetermined polarization mode and another polarization mode, respectively.
However, the above conventional oscillation polarization mode selective DFB semiconductor laser, which selects the lapsing polarization mode depending on the phase condition, is sensitive to the phase at the end facet. As a result, the lapsing wavelength and polarization mode of the device depend on the current injection condition in a complicated fashion, and fluctuation in characteristics of the lapsing polarization mode appears among individual devices. In particular, regarding the fluctuation among devices, it is difficult to selectively impart a gain to one of the competing polarization modes when a current injected into one of two waveguide portions is increased, so that a lapsing polarization switching point varies among the devices.
Further, in the conventional laser, the phase relations of resonant light in the cavity for respective polarization modes are controlled by the balance of currents injected into a uniform structure extending in the cavity direction (a uniform active layer and a uniform diffraction grating), and the lapsing polarization mode is thus switched by changing the polarization mode whose oscillation threshold gain is the smallest. Therefore, the conventional laser suffer the following disadvantages.
(1) Since the light phase is controlled, the lapsing polarization mode is again returned to the TE mode, for example, when the switching from the TE mode to the TM mode is conducted by increasing a current injected into a certain region and thereafter this current is further increased.
(2) Due to the multi-electrode structure, a common polarization mode is not selected among plural regions when currents injected into those regions increase. The respective regions are thus brought into independent lapsing conditions, and those conditions influence each other to cause a plurality of longitudinal modes.
On the other hand, in the above another conventional DFB semiconductor laser, the light wave in the predetermined polarization mode is generated or amplified by the choice of its geometrical shape, so that its yield varies due to processing fluctuations of etching depth and edge width during its ridge fabrication process.
An object of the present invention is to provide a distributed feedback semiconductor laser which can simplify its stimulated condition (typically, its current injection condition) for causing the switching of its oscillation or lapsing polarization mode, a method for driving the semiconductor laser, a light source apparatus which can perform the modulation with a large extinction ratio using the semiconductor laser, an optical transmission method using the semiconductor laser, an opto-electric converting apparatus suitably usable in the optical transmission method, an optical transmission system using the opto-electric converting apparatus, and so forth.
A distributed feedback semiconductor laser of the present invention includes a cavity extending in a cavity-axial direction and including a plurality of regions, and a plurality of waveguides with a diffraction grating and an active layer extending along the cavity-axial direction. The waveguides are formed in the regions, respectively, coupled to each other along the cavity-axial direction, and define different first and second polarization modes (typically, TE mode and TM mode). The semiconductor laser further includes a light intensity distribution control portion formed in the cavity and having a function to relatively and locally strengthen light intensity distributions for the first and second polarization modes in the cavity-axial direction with a polarization dependency, and a control unit (typically, a current injection unit) for independently stimulating at least two of the regions.
The following specific structures are possible based on the above fundamental structure.
The light intensity distribution control portion imparts a larger action, which relatively strengthens a light intensity distribution at a place of the light intensity distribution control portion in the cavity, to one of the first and second polarization modes, as compared to the other of the first and second polarization modes. More specifically, the light intensity distribution control portion imparts substantially no action to the other of the first and second polarization modes.
The light intensity distribution control portion may comprise a phase shift section for shifting a phase of a periodical change in a refractive index for one of the first and second polarization modes due to the diffraction grating while not shifting a phase of a periodical change in a refractive index for the other of the first and second polarization modes due to the diffraction grating. Further, the phase shift section may shift the phase of the periodical change in the refractive index for one of the first and second polarization modes by 180 degrees. The phase shift portion shifts the phase for one polarization mode such that its light intensity is strengthened at a place of the shift section, while shifting the phase for the other polarization mode only to such a degree that its light intensity is strengthened less than the strengthening degree for one polarization mode. Preferably, the phase shift section imparts almost no phase shift to the other polarization mode.
In the above structure, the light intensity distribution control portion primarily sensed only by one polarization mode has a function to fixedly strengthen the light intensity for this mode at the place of this distribution control portion. In contrast, the other polarization mode comes to have a light intensity distribution different from that for the one polarization mode since the other mode cannot sense the function of the light intensity distribution control portion. In the specific structure, the phase shift section primarily sensed by one polarization mode has an action to cause oscillation at a center wavelength of the stopband in this mode and fixedly strengthen its light intensity at a place of the phase shift section. The other polarization mode has a light intensity distribution different from that for the one polarization mode since one mode cannot sense the phase shift action.
The light intensity distributions in the cavity-axial direction thus comes to be different between two competitive polarization modes. Therefore, independent current injections into the two regions are classified into one for inducing the oscillation in the TE mode and one for inducing the oscillation in the TM mode, for example. Hence, the switching of the lapsing polarization mode can be readily and stably performed by the control unit.
In one of typical configurations of the DFB semiconductor laser of the present invention, periodical striped grooves formed in a birefringence layer with a birefringence of nlTE (a refractive index for TE-mode light) greater than nlTE (a refractive index for TM-mode light) are buried with a layer with a refractive index of nb satisfying the relation nlTE greater than nb greater than nlTM, and a portion, in which periodical striped grooves formed in a layer with a refractive index of ng satisfying the relation ng less than nb are buried with a layer with a refractive index of nb, are serially arranged in the cavity-axial direction to form the phase shift section for shifting the phase only for the TE-mode light at an interface between the two portions in the diffraction grating. In this structure, the relation (nbxe2x88x92ng) greater than (nbxe2x88x92nlTM) is preferably satisfied.
The operation of the first typical configuration will be described with reference to FIGS. 1A to 1C. The phase of the diffraction grating sensed by the TE-mode light will be described first. In one portion (a portion on the right side of a phase shift section S in FIG. 1A, but this portion does not coincide with a region into which a current can be independently injected), the refractive index nlTE of a layer 107 with periodical stripes is larger than the refractive index nb of a burying layer 108, so that up portions of the periodical stripes correspond to up portions of a periodical change in the effective refractive index sensed by the TE-mode light. In the other portion (a portion on the left side of the section S in FIG. 1A), the refractive index ng of a layer 106 with periodical stripes is smaller than the refractive index nb of the burying layer 108, so that up portions of the periodical stripes correspond to down portions of a periodical change in the effective refractive index sensed by the TE-mode light. Therefore, the up and down change in the refractive index sensed by the TE-mode light is inverted at the phase shift section S relatively to the up and down change of the periodical stripes. This inversion serves as a phase shift in the diffraction grating.
The phase of the diffraction grating sensed by the TM-mode light will be described. In both the two portions, the refractive index nlTM of the layer with periodical stripes is smaller than the refractive index nb of the burying layer 108, so that no phase shift exists in the diffraction grating. Further, when the relation of (nbxe2x88x92ng) greater than (nbxe2x88x92nlTM) exists, a difference between up and down portions of the refractive index sensed by the TM-mode light in the portion on the left side of the section S is larger than that in the portion on the right side of the section S. Thus, the light intensity distributions as illustrated in FIG. C are established.
In the second typical configuration of the DFB semiconductor laser, a portion, in which periodical 5 striped grooves formed in a birefringence layer with a birefringence of nlTE (a refractive index for TE-mode light) greater than nlTM (a refractive index for TM-mode light) are buried with a layer with a refractive index of nb satisfying the relation nlTE greater than nb greater than nlTM, and a portion, in which periodical striped grooves formed in a layer with a refractive index of ng satisfying the relation ng greater than nb are buried with a layer with a refractive index of nb, are serially arranged in the cavity-axial direction to form the phase shift section for shifting the phase only for the TM-mode light at an interface between the two portions in the diffraction grating. In this structure, the relation (ngxe2x88x92nb) greater than (nlTExe2x88x92nb) is preferably satisfied.
In the second typical configuration, the refractive index relation between the periodical stripes and the burying layer in the other portion is opposite to that of the first typical configuration, so the phase shift is present only for the TM-mode light.
In the typical configurations, more specifically, a pitch of the periodical stripes or the diffraction grating is uniform in the two portions. Further, the birefringence layer may be a multiple quantum well layer. In this case, the birefringence layer can be highly controllably formed using ordinary crystalline growth techniques. Furthermore, the phase shift section may be located around a central portion of one of the regions. Thereby, one of two electrodes serving as the control unit can correspond to a portion where a light intensity distribution for a predetermined polarization mode is large, so the mode switching can be readily carried out.
More specifically, the active layer may be an active layer with approximately equal gains for the TE-mode light and the TM-mode light common to the regions. In this structure, contribution of the polarization dependency in gains of the active layer to the mode switching characteristics can be suppressed, so that the mode switching can be readily effected by controlling currents injected into the regions.
Further, a high-reflection film may be provided on an end facet of an outermost region of the regions. In particular, the high-reflection film determines the light intensity distribution in the cavity for the mode light which cannot sense the action of the light intensity distribution control portion. The high-reflection film is preferably provided on the end facet of the outermost region without the light intensity distribution control portion. The high-reflection film fixedly brings a strong portion of the light intensity distribution for the polarization-mode light, which cannot sense the action of the light intensity distribution control portion, into the region lacking the distribution control portion. Thus, another light intensity distribution control portion is provided such that the light intensity distribuitons can have a more preferable polarization dependency.
In a driving method of the present invention, the above semiconductor laser is brought into an oscillation condition in the first polarization mode by injecting currents into at least two regions, and the current injected into one of the regions, whose electric-field or light intensity distribution for the second polarization mode is larger than its electric-field intensity distribution for the first polarization mode, is relatively increased to stably switch the oscillation condition in the first polarization mode to an oscillation condition in the second polarization mode. A uniform current can be injected into the regions by the control unit to bring the semiconductor laser into the oscillation condition in the first polarization mode. Further, the current injected into one of the regions, whose light intensity distribution for the first polarization mode is larger than its light intensity distribution for the second polarization mode, is relatively increased to stably switch the oscillation condition in the second polarization mode to the oscillation condition in the first polarization mode.
This driving method uses the property that when the current injected into the region whose field intensity distribution for a desired polarization mode is large is relatively increased, the lapsing polarization mode can be readily and stably switched to this desired polarization mode.
These advantages and others will be more readily understood from the following detailed description of the preferred embodiments in conjunction with the drawings.